The Vieux Carré serves as an excellent example of what it means to riff on a classic cocktail. This drink is effectively a Manhattan, in which each of the main ingredients has been split amongst two different spirits: bourbon or rye gives up some of its real estate to cognac, vermouth offers to share its seat with an herbal liqueur, and one type of bitters is replaced by two. This strategy is a simple but effective one to use when building a riff on your own favorites.